Prior art monitoring apparatuses of the type mentioned above simply judge the occurrence of a disturbance by mechanical comparison of the detected data with a single fixed reference level and automatically issue an alarm. However, the environmental conditions of the places where the detectors are installed differ from place to place. Also the characteristics of the detectors may vary from detector to detector because of minute difference in the product quality and aging. Therefore, prior art monitoring apparatuses as mentioned above often issue false alarms or fail to issue alarms when required.
It is possible to use more complicated variables as the indication of disturbances. For instance, there are apparatuses in which amount of change in a fixed period of time is measured and compared with a reference. It is possible to employ a more complicated reference instead of a fixed level. For instance, the average or integral of data collected over a fixed period of time preceding the judgment can be employed as a reference. However judgment based on a complicated variable or a complicated reference will require a complicated algorithm which means that an expensive processing unit is required. It is not practical to provide such an expensive processing unit for each detector. Therefore, judgment is usually made by a single processing unit provided with a single reference for all the detectors. That is, it is impossible to take into consideration the individual characteristics of each sensor such as the time course change in sensitivity and the conditions of the place where the detector is installed.
Therefore, it is the object of this invention to overcome the above-stated drawback of prior art apparatuses. That is, the object of this invention is to provide an apparatus for monitoring environmental disturbances which enables the operator to determine the occurrence of a disturbance taking into consideration the individual characteristics of each detector and the conditions of the place where each detector is installed.